Method for treating an exhausted glycol-based slurry

ABSTRACT

A method of separating, recovering and reusing components of an exhausted slurry used in slicing silicon wafers from a silicon ingot. In the method, the solid particles and lubricating fluid of the exhausted slurry are separated without decreasing the viscosity of the exhausted slurry. The separated lubricating fluid may be collected and reused in the preparation of a fresh slurry. Additionally, the silicon particulate and metal slicing wire particulate are dissolved and separated from the abrasive grains. The abrasive grains are separated into spent abrasive grains and unspent abrasive grains. The separated unspent abrasive grains are suitable for reuse in the preparation of a fresh slurry.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method for the regenerationof an abrasive slurry utilized in the preparation of silicon wafers.More particularly, the present invention relates to a method ofseparating components of an exhausted slurry, used in slicing wafersfrom a single crystal or polycrystalline silicon ingot, such that thedesirable abrasive grains and lubricating, or cooling, fluid therein maybe reused.

Silicon wafers are obtained from a single crystal or polycrystallinesilicon ingot by first slicing the ingot in a direction normal to itsaxis. Typically, the slicing operation is accomplished by means of awire saw, wherein the silicon ingot is contacted with a reciprocatingwire while a slurry containing abrasive grains is supplied to thecontact area between the ingot and the wire. Conventional wire sawslurries typically comprise a lubricating, or cooling fluid, such as,for example, mineral oil or some water soluble liquid (e.g.,polyethylene glycol, or “PEG”), and abrasive grains, such as siliconcarbide.

During the slicing operation, silicon particulate from the ingot andmetal particulate (primarily iron) from the slicing wire and metal pipesform and are incorporated into the slurry. As the concentrations of thesilicon and metal particulate in the slurry increase, the efficiency ofthe slicing operation decreases. Eventually, the slurry becomesineffective, or “exhausted,” and must be discarded. Typically, theexhausted slurry is disposed of by incineration or treated by a wastewater treatment facility. However, burning this slurry generates carbondioxide and sending this slurry to a waste water treatment facilitytypically results in the formation of a sludge which must be disposed ofin a landfill. Accordingly, both approaches of disposal are unfavorablefrom an environmental point of view. It is therefore desirable toeliminate, or significantly reduce the amount of, this waste which isgenerated.

In addition to the negative environmental consequences and the costsassociated with the disposal of the exhausted slurry, silicon waferproduction costs are also increased due to the fact that the slurry isgenerally disposed of prematurely. More specifically, the rate at whichthe abrasive grains in the slurry are “spent” (i.e., worn to a size toosmall for effective slicing) by the slicing operation is generally muchless than the rate at which the slurry becomes contaminated by siliconand metal particulate. Furthermore, the useful life of a typicallubricating or cooling fluid is dictated almost entirely by the build-upof silicon and/or metal particulate; that is, the lubricating or coolingsolution could be used for a much longer period of time, if it were notfor the increasing concentration of silicon and metal particulate in theslurry. As a result, the slurry is typically discarded once the level ofsilicon and/or metal particulate in the slurry is too high, even thoughmuch of the abrasive grains and lubricating fluid are still usable.

In view of the foregoing, a need continues to exist for a method whichmay be utilized to separate silicon and metal wire particulate from theslurry, thus enabling the reuse of abrasive grains and the lubricatingfluid. Such a process would reduce the manufacturing costs associatedwith the slicing of silicon ingots. In addition, such a process wouldreduce the amount of waste product and/or waste byproduct emitted intothe environment.

SUMMARY OF THE INVENTION

Among the several objects and features of the present invention may benoted the provision of a method for extending the useful life of thecomponents of a slurry utilized in slicing a silicon ingot; theprovision of such a method wherein these components are regenerated byseparating them from an exhausted slurry; the provision of such a methodwherein the lubricating fluid may be efficiently regenerated byseparating it from solids in the slurry; the provision of such a methodwherein unspent abrasive grains may be regenerated by separating themfrom spent abrasive grains, silicon particulate and metal particulate;the provision of such a method wherein the regenerated lubricating fluidand the regenerated abrasive grains are reintroduced into the siliconwafer manufacturing process; the provision of such a method wherein theliquid (such as water) used in the separation of solids is reused; theprovision of such a method wherein the separated spent abrasive grainsmay be reused (e.g., in the manufacture of grinding wheels or cement);the provision of such a method wherein the soluble silicon saltbyproduct and the etchants used in the method are reused (e.g., in thepreparation of industrial soaps); the provision of such a method whereinthe amount of waste which must be discarded is decreased or eliminated;and, the provision of such a method wherein the cost of producingsilicon wafers is reduced.

Briefly, therefore, the present invention is directed to a method fortreating an exhausted slurry utilized in slicing silicon wafers from asilicon ingot, the exhausted slurry comprising a lubricating fluid,unspent abrasive grains, spent abrasive grains, silicon particulate andmetal particulate. The exhausted slurry is separated into a first liquidfraction and a first solids fraction, the first liquid fractioncomprising lubricating fluid, the first solids fraction comprisingunspent abrasive grains, spent abrasive grains, silicon particulate andmetal particulate. The first solids fraction is mixed with a firstetchant to form a first solid/etchant mixture in which at least one ofthe group consisting of silicon particulate and metal particulate isdissolved.

In another embodiment the exhausted slurry is separated into a firstliquid fraction and a first solids fraction, the first liquid fractioncomprising lubricating fluid suitable for direct incorporation into afresh slurry, the first solids fraction comprising unspent abrasivegrains, spent abrasive grains, silicon particulate and metalparticulate. The first solids fraction is mixed with water to form anaqueous suspension comprising unspent abrasive grains, spent abrasivegrains, silicon particulate and metal particulate. The aqueoussuspension is separated into a recycle fraction and waste fraction, therecycle fraction comprising unspent abrasive grains, siliconparticulate, metal particulate and water, the waste fraction comprisingspent abrasive grains, silicon particulate, metal particulate and water.The water is separated from the recycle fraction and then the recyclefraction is mixed with a first etchant to form a first solid/etchantmixture in which the silicon particulate or the metal particulate isdissolved. The first solid/etchant mixture is separated into a secondliquid fraction and a second solids fraction, the second liquid fractioncomprising the first etchant and dissolved silicon particulate ordissolved metal particulate, the second solids fraction comprisingunspent abrasive grains and silicon particulate or metal particulate notdissolved in the first etchant. The second solids fraction is mixed witha second etchant to form a second solid/etchant mixture in which thesilicon particulate or the metal particulate is dissolved. The secondsolid/etchant mixture is separated into a third liquid fraction and athird solids fraction, the third liquid fraction comprising the secondetchant and dissolved silicon particulate or dissolved metalparticulate, the third solids fraction comprising unspent abrasivegrains suitable for direct incorporation into a fresh slurry.

In a further embodiment the exhausted slurry is separated into a firstliquid fraction and a first solids fraction, the first liquid fractioncomprising lubricating fluid, the first solids fraction comprisingunspent abrasive grains, spent abrasive grains, silicon particulate,metal particulate and lubricating fluid. The first solids fraction iswashed with a solvent to reduce the concentration of lubricating fluidin the first solids fraction and form a wash liquor comprising solventand lubricating fluid. The wash liquor is added to the first liquidfraction and the first liquid fraction is heated to a temperatureranging from about 50° C. to about 100° C. to evaporate the solventuntil the first liquid fraction is suitable for direct incorporationinto a fresh slurry. The washed first solids fraction is mixed with anaqueous sodium hydroxide solution to form a first solid/etchant mixturein which the silicon particulate is dissolved. The first solid/etchantmixture is separated into a second liquid fraction and a second solidsfraction, the second liquid fraction comprising the aqueous sodiumhydroxide solution and dissolved silicon particulate; the second solidsfraction comprising unspent abrasive grains, spent abrasive grains andmetal particulate. The second solids fraction is mixed with an aqueoussulfuric acid solution to form a second solid/etchant mixture in whichthe metal particulate is dissolved. The second solid/etchant mixture isseparated into a third liquid fraction and a third solids fraction, thethird liquid fraction comprising the aqueous sulfuric acid solution anddissolved metal particulate, the third solids fraction comprisingunspent abrasive grains and spent abrasive grains. The third solidsfraction is rinsed with water and then dried until the water content isless than about 1000 ppm, the dried third solids fraction being suitablefor direct incorporation into a fresh slurry provided the concentrationof spent abrasive grains is less than about 7 percent by weight of thethird solids fraction.

In a preferred embodiment the exhausted slurry is separated into a firstliquid fraction and a first solids fraction, the first liquid fractioncomprising lubricating fluid, the first solids fraction comprisingunspent abrasive grains, spent abrasive grains, silicon particulate,metal particulate and lubricating fluid. The first solids fraction iswashed with a solvent to reduce the concentration of lubricating fluidin the first solids fraction and form a wash liquor comprising solventand lubricating fluid. The wash liquor is added to the first liquidfraction and the first liquid fraction is heated to a temperatureranging from about 50° C. to about 100° C. to evaporate the solventuntil the first liquid fraction is suitable for direct incorporationinto a fresh slurry. The washed first solids fraction is mixed with anaqueous sodium hydroxide solution to form a first solid/etchant mixturein which the silicon particulate is dissolved. The first solid/etchantmixture is separated into a second liquid fraction and a second solidsfraction, the second liquid fraction comprising the aqueous sodiumhydroxide solution and dissolved silicon particulate; the second solidsfraction comprising unspent abrasive grains, spent abrasive grains andmetal particulate. The second solids fraction is mixed with an aqueoussulfuric acid solution to form a second solid/etchant mixture in whichthe metal particulate is dissolved. The second solid/etchant mixture isseparated into a third liquid fraction and a third solids fraction, thethird liquid fraction comprising the aqueous sulfuric acid solution anddissolved metal particulate, the third solids fraction comprisingunspent abrasive grains and spent abrasive grains. The third solidsfraction is mixed with water to form an aqueous suspension comprisingunspent abrasive grains and spent abrasive grains. The aqueoussuspension is separated into a recycle fraction and waste fraction, therecycle fraction comprising unspent abrasive grains and water, the wastefraction comprising spent abrasive grains and water. The recyclefraction is separated into a reusable suspending liquid and isolatedunspent abrasive grains. The isolated unspent abrasive grains are drieduntil the water content is less than about 1000 ppm, the dried isolatedunspent abrasive grains being suitable for direct incorporation into afresh slurry.

Other objects and features of the present invention will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow chart which details a portion of the steps, some ofwhich are optional, that are employed in accordance with the presentinvention to isolate desirable components of the exhausted slurry.Specifically, FIG. 1 is directed to the separation and recovery oflubricating fluid.

FIG. 2 is a flow chart which details a portion of the steps, some ofwhich are optional, that are employed in accordance with the presentinvention to isolate desirable components of the exhausted slurry.Specifically, FIG. 2 is directed to the separation of silicon and metalparticulate.

FIG. 3 is a flow chart which details a portion of the steps, some ofwhich are optional, that are employed in accordance with the presentinvention to isolate desirable components of the exhausted slurry.Specifically, FIG. 3 is directed to the separation and recovery ofsilicon carbide abrasive grains.

DETAILED DESCRIPTION OF THE INVENTION

In order to reduce the amount of waste generated by standard siliconwafer production processes, as well as reduce the costs associated withsilicon wafer production in general, it is desirable to regenerate orrecycle the exhausted abrasive slurry used to slice the silicon ingotsinto wafers. More specifically, it is desirable to isolate thosecomponents of the slurry which are still suitable for purposes ofslicing an ingot (i.e., the lubricating fluid and the unspent abrasivegrains) from the remainder of the slurry and use the isolated desirablecomponents to prepare fresh slurries. The components of the exhaustedabrasive slurry not reused to prepare fresh slurries (i.e., the spentabrasive grains, the silicon particulate and the metal particulate) maybe used in other applications.

As used herein, “exhausted slurry” refers to a slurry which isessentially no longer suitable for purposes of slicing silicon wafersfrom a silicon ingot as a result of, for example, an unacceptably highcontent of silicon and/or metal particulate that hinders the slicingoperation. It is presently believed that silicon particulate hinders asilicon ingot slicing operation at concentrations above about 1–5% byweight, preferably above about 3% by weight of the solid matter in theslicing slurry. It is also believed that metal particulate hinders theslicing operation at concentrations about about 0.5–2%, preferably aboveabout 1% by weight. “Spent abrasive grains” refers to abrasive grainswhich, as a result of being worn down by the slicing process, are of adiameter or size which is generally no longer suitable for purposes ofslicing silicon wafers from a silicon ingot. It is presently believedthat abrasive grains are spent at less than about 1 μm in size. “Unspentabrasive grains” refers to abrasive grains in the exhausted slurry whichare still suitable for purposes of slicing silicon wafers from a siliconingot (presently believed to be greater than about 1 μm in size). It ispresently believed that the spent abrasive grains hinder a silicon ingotslicing operation at concentrations above about 5–10%, and preferablyabove about 7%, by weight of the total abrasive grains (i.e., spent andunspent abrasive grains). “Viscosity” refers to the dynamic viscosity ofthe slurry, which is the viscosity as determined or measured while theslurry is in flow, by means common in the art.

The method of the present invention separates waste solids (i.e.,silicon particulate, metal particulate and spent abrasive grains) fromthose slurry components which may be reused (i.e., lubricating fluid andunspent abrasive grains) by (1) segregating the suspended solids andlubricating fluid, (2) segregating the abrasive grains (spent andunspent) and the silicon and metal particulate and (3) segregating thespent and unspent abrasive grains.

A. Segregation of Suspended Solids and Lubricating Fluid

Referring generally to FIG. 1, in accordance with the present method thesolid matter in the slurry is separated from the lubricating fluid. Anymethod for separating solids of a very small diameter (e.g., about 5 μm,3 μm, 1 μm or even smaller) may be used. Preferably, the method employedwill be one which yields lubricating fluid that is substantially free ofsolids (e.g., preferably less than about 1 g of solids per liter oflubricating fluid, more preferably less than about 0.5 g/liter and evenmore preferably less than about 0.3 g/liter). Filtration of theexhausted slurry, and more preferably press filtration, is an example ofthe method to employ to separate the solid matter from the lubricatingfluid. Press filtration generally involves separating the exhaustedslurry into a liquid fraction and a solids fraction by passing theexhausted slurry through at least one screen, such as a polypropylenescreen, having a pore or mesh size which is sufficient to removesubstantially all of the solids from the fluid at an elevated pressure.An exemplary press filtration device is the model de800/59ppmanufactured by Diefenbach (Bergamo, Italy). The press filtration may beperformed by passing about 300 to about 700 liters per hour of theexhausted slurry and preferably about 500 liters per hour of theexhausted slurry at a pressure of about 115 psi (about 0.8 MPa) to about220 psi (about 1.5 MPa) and preferably at about 145 psi (about 1.0 MPa)to about 175 psi (about 1.2 MPa) through a screen having a pore or meshsize of less than about 5 μm, preferably less than about 3 μm, and morepreferably less than about 1 μm.

Typically, filtration yields a “cake” of solids fraction comprisingsilicon particulate, metal particulate, spent abrasive grains, unspentabrasive grains and lubricating fluid (the concentration of lubricatingfluid in the cake is typically less than about 25% by weight of thecake, preferably less than about 20%, more preferably less than about15% and even more preferably less than about 10%). The solids fractionresulting from the separation is further processed, in order to isolateunspent abrasive grains from the silicon particulate, metal particulateand spent abrasive grains. Preferably, however, prior to separating thesolids, and while in the filtering apparatus, the solids fraction iswashed with a solvent (e.g., water or methanol, preferably water) toreduce the concentration of lubricating fluid. After being washed, theconcentration of lubricating fluid in the cake is typically less thanabout 5% by weight of the cake, preferably less than about 2% and morepreferably less than about 1%. The solvent and lubricating fluid washedfrom the solids fraction (“wash liquor”) are preferably added to thepreviously separated liquid fraction prior to further processing of theliquid fraction.

The liquid fraction resulting from the separation is substantially freeof solids and may be directly recycled into the silicon wafermanufacturing process, for use as the lubricating or cooling fluid ofthe silicon ingot slicing slurry without any additional separation orprocessing steps being performed. A residual amount of solid particles,up to about 0.5 grams/liter, typically remains in the liquid fraction.Typically, the residual solid particles are primarily silicon and areless than about 0.3 μm in diameter. However, if desired, an optional“clarification” step may be performed, wherein the fraction is subjectedto an additional separation step to reduce the concentration of theresidual solid particles. Any method for separating solids of about 0.1μm or less may be used, preferably press filtration as described above.After clarification, the solids content of the liquid fraction istypically reduced below about 1 part per billion by weight.

As described above, the method of the present invention allows theliquid fraction to be recycled after separation or clarification.However, if the wash liquor is added to the liquid fraction, at least aportion of the solvent is preferably separated from the liquid fractionusing evaporation or distillation. Preferably, the evaporation of thesolvent is accomplished by heating the fraction. In the case of a liquidfraction comprising a “glycol”-based lubricating fluid, it is preferablyheated to a temperature of about 50° C. to about 100° C., morepreferably about 80° C. To enhance evaporation, the pressure to whichthe fraction is subject may be reduced, but atmospheric pressure ispreferable so that the complexity and cost of the process is minimized.Evaporation increases the concentration of lubricating fluid in thefraction and is continued until the lubricating fluid reaches aviscosity suitable for the preparation of fresh slurry. For example,prior to concentration, a fraction comprising a “glycol”-basedlubricating fluid and water typically has a viscosity of about 1 cps toabout 2 cps at about 70° C. to about 90° C. and evaporation is continueduntil the viscosity of the fraction is about 10 cps to about 15 cps atabout 70° C. to about 90° C. After cooling to about 25° C., theviscosity of the isolated lubricating fluid (i.e., the increasedviscosity fraction) is preferably about 90 cps to about 120 cps and mayused in the preparation of fresh slurry.

B. Segregation of Abrasive Grains and Silicon & Metal Particulate

The solids fraction separated from the exhausted slurry is furtherprocessed to isolate spent and unspent abrasive grains from siliconparticulate and metal particulate. To facilitate the isolation of theabrasive grains, the solids fraction (cake) from the initial separationis mixed with at least one etchant to dissolve the silicon particulateand the metal particulate. For example, the solids fraction may be mixedwith an etchant capable of dissolving both silicon particulate and metalparticulate. One such etchant comprises hydrofluoric acid (HF) andnitric acid (HNO₃) typically in relatively high concentrations (e.g., HFand HNO₃ each typically comprise about 10 to about 20% by weight of thesolution and preferably about 15% by weight of the solution). Highlyconcentrated HF-HNO₃ etchants, however, are hazardous and requireextensive safety precautions. As such, the use of less hazardousetchants is preferred. Typically, this requires the use of an etchant todissolve the silicon particulate and a different etchant to dissolve themetal particulate. The order in which the silicon and metal particulateare dissolved is not significant, however, the following discussion andthe flow chart of FIG. 2 provide for the silicon particulate to bedissolved before the metal particulate.

1. Separation of the Silicon Particulate

Referring generally to FIG. 2, the solids fraction from the initialseparation is mixed with an etchant to form a solid/etchant mixture inwhich silicon particulate is dissolved. Preferably, the etchant is anaqueous solution comprising an alkali metal hydroxide (e.g., NaOH andKOH) and/or an alkaline earth metal hydroxide (e.g., Ca(OH)₂). Morepreferably, the etchant is an aqueous solution comprising sodiumhydroxide at a concentration of from about 6% to about 20% by weight,and more preferably from about 8% to about 15% by weight. Typically, atleast about 6 liters of sodium hydroxide solution is added per kilogramof solids fraction, preferably at least about 7 liters, and morepreferably at least about 8 liters. Most preferably, the amount ofetchant added to the solid material is merely sufficient to dissolveessentially all of the silicon particulate.

The dissolution of silicon by a basic etchant results in the formationof a soluble alkali/alkaline earth silicon salt (e.g., Na₂SiO₃) andhydrogen gas. The hydrogen gas may be collected and used in otherapplications.

2. Separation of Solids from Etchants

Following the dissolution of silicon particulate, the remaining solidmatter is separated from the etchant comprising the dissolved siliconparticulate in the form of a soluble salt. Preferably, the solid/etchantmixture is passed through the press filter used previously to separatethe solid fraction from the lubricating fluid thereby reducing theequipment costs associated with the present regeneration method. Thecake of filtered solids comprises spent abrasive grains, unspentabrasive grains, metal particulate and a residual amount of etchant. Ifdesired, the amount of etchant in the cake may be further reduced bydiluting the cake with water and refiltering the mixture. Typically,after the dilution and filtration the concentration of the etchant inthe cake is less than about 0.1% by weight.

3. Separation of the Metal Particulate

After separating the silicon particulate, the solids fraction is furtherprocessed to separate the unspent abrasive grains and the spent abrasivegrains from the metal particulate by mixing the solids fraction with anetchant that dissolves the metal particulate. Preferably, the etchant isselected from the group consisting of an aqueous sulfuric acid solution,an aqueous nitric acid solution and mixtures thereof. More preferably,the etchant is an aqueous sulfuric acid solution. Preferably, theconcentration of the sulfuric acid in etchant is from about 0.3% toabout 1% by weight and more preferably from about 0.4% to about 0.8% beweight. Typically, at least about 6 liters of the sulfuric acid solutionis added per kilogram of solids fraction, preferably at least about 7liters, and more preferably at least about 8 liters. Most preferably,the amount of etchant added to the solid material is merely sufficientto dissolve essentially all of the metal particulate.

The metals dissolved with the acid solution form soluble salt compoundssuch as iron sulfates (e.g., Fe₂(SO₄)₃ and FeSO₄), copper sulfates andzinc sulfates which are filtered from the solids fraction as set forthabove. The remaining solids fraction comprises spent abrasive grains andunspent abrasive grains. Any residual acidic etchant may be furtherreduced by diluting the cake with water and re-filtering the solids.

C. Segregation of Spent and Unspent Abrasive Grains

As discussed above, the slicing slurry typically becomes exhausted bymetal or silicon particulate at a faster rate than by spent abrasivegrains, thus, the segregation of spent and unspent abrasive grains neednot be performed as often as the segregation metal and siliconparticulate. Typically, the spent and unspent abrasive grains areseparated when the concentration of spent abrasive grains exceeds about5–10% by weight and preferably about 7% by weight of the total abrasivegrains which corresponds to an approximate 1 to 2 μm shift in theparticle size distribution of the abrasive grains.

The separation of spent and unspent abrasive grains can be performed atany point in the method of the present invention after segregation ofthe solids fraction (comprising spent and unspent abrasive grains andsilicon and metal particulate) and the liquid fraction (comprisinglubricating fluid). For example, the separation of spent and unspentabrasive grains may be performed prior to dissolving silicon particulateand metal particulate, after dissolving either silicon particulate ormetal particulate, or after dissolving both silicon particulate andmetal particulate. Preferably, the spent and unspent abrasive grains areseparated after silicon particulate and metal particulate are dissolved.

The separation can be accomplished by any means common in the art whichis capable of separating particles based on weight or size, such as, forexample, a hydro-cyclone separator, a sedimentation centrifuge, afiltration centrifuge or a filter (in conjunction with a filter clothhaving the appropriate pore size for the necessary selectivity).Preferably, this separation is accomplished by means of a hydro-cycloneseparator, which is preferred due to its relatively low cost anddurability (e.g., model number RWK810 by AKW of Germany).

Referring generally to FIG. 3, the remaining solids fraction (i.e.,spent and unspent abrasive grains) are mixed with a liquid such aswater, an acid etchant or a basic etchant to form an aqueous suspension(thus, the dissolution of the silicon and/or metal particulate may beincorporated with the separation of the unspent and spent abrasivegrains). Preferably, water is mixed with the solids fraction and theconcentration of solid particles is about 5 grams/liter to 100grams/liter, preferably about 15 grams/liter to about 60 grams/liter,more preferably about 25 grams/liter to about 30 grams/liter. Generallyspeaking, in a hydro-cyclone separator, water and solid particlesweighing less than a predetermined amount are discharged from an upperoutlet while liquid and particles equal to or heavier than thepredetermined weight are discharged from a lower outlet. In the presentinvention, the predetermined weight corresponds to particles of a sizewhich allows separation of the unspent abrasive grains from the spentgrains. Accordingly, after hydro-cyclone separation two suspensionsremain—a recycle fraction comprising unspent abrasive grains and waterand a waste fraction comprising spent abrasive grains and water.

If the separation of spent and unspent abrasive grains is performedbefore the silicon and/or metal particulate are separated, in additionto the spent abrasive grains, the waste fraction also comprises asignificant portion of the silicon and metal particulate. This allows asignificant reduction in the amount etchant(s) needed to separate anysilicon and/or metal particulate remaining in the recycle fraction withthe unspent abrasive grains, thereby reducing raw material costs.

D. Drying the Unspent Abrasive Grains

The liquid is removed from the recycle fraction to isolate the unspentabrasive grains for reuse in the wafer manufacturing process. Theunspent abrasive grains may be isolated from the recycle fraction bymeans common in the art, such as by filtration or centrifugation.Preferably, a belt and vacuum filter or a filter press is used to removethe excess liquid. After the unspent abrasive grains are isolated fromthe first suspension, the grains may be reused to create fresh wireslicing slurry and the liquid (e.g., water) may be reused in the methodof the present invention. Preferably, however, the unspent abrasivegrains are dried by any means common in the art, such as by placing thefiltered solids in an oven. It is preferable that the collected grainsbe periodically moved during the drying process due to the tendency ofsilicon carbide abrasive grains to form relatively large masses ofsolidified material.

It is to be noted that when heat is applied to dry the unspent abrasivegrains it is preferable to use a temperature which will dry the solidsas quickly and as efficiently as possible. Accordingly, preferably theabrasive grains will be dried at a temperature between about 75° C. andabout 200° C., and more preferably between about 100° C. and about 150°C. Although the drying time may vary, depending upon, for example, theliquid being removed and the method of drying, typically drying willcontinue until only a trace amount, or less, of the liquid remains. Forexample, if water is used to form the solid/liquid mixture, typicallythe abrasive solids will be dried until the water content is less thanabout 5000 ppm (parts per million), preferably less than about 1000 ppm,and most preferably less than about 500 ppm, as determined by meanscommon in the art (such as by the Karl Fischer method).

A low water content is desirable because the presence of water acts todecrease the viscosity of the slurry in which the recovered abrasivegrains are used. Furthermore, the presence of water in the slurry cancause the abrasive grains, such as silicon carbide, to stick together,resulting in the formation of larger abrasive grains which can damagethe surface of the wafer during the slicing process.

The waste fraction (i.e., the suspension containing the spent abrasivegrains), may be discarded. Preferably, however, the waste fraction willbe concentrated and the liquid, such as water, will be collected andreused in the method of the present invention to reduce the amount ofwaste generated. The suspended spent abrasive grains may optionally becollected and dried, as well.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained. It isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

1. A method for treating an exhausted slurry utilized in slicing silicon wafers from a silicon ingot, the exhausted slurry comprising a lubricating fluid, unspent abrasive grains, spent abrasive grains, silicon particulate and metal particulate, the method comprising: (a) separating the exhausted slurry into a first liquid fraction and a first solids fraction, the first liquid fraction comprising lubricating fluid, the first solids fraction comprising unspent abrasive grains, spent abrasive grains, silicon particulate and metal particulate; and (b) mixing the first solids fraction with a first etchant to form a first solid/etchant mixture in which at least one of the group consisting of silicon particulate and metal particulate is dissolved.
 2. The method as set forth in claim 1 wherein step (a) is conducted in a manner such that the first liquid fraction is suitable for direct incorporation into a fresh slurry.
 3. The process as set forth in claim 1 wherein the lubricating fluid is polyethylene glycol.
 4. The method as set forth in claim 1 wherein the first solids fraction comprises lubricating fluid and the method further comprises washing the first solids fraction with a solvent to reduce the concentration of lubricating fluid in the first solids fraction and form a wash liquor comprising solvent and lubricating fluid.
 5. The method as set forth in claim 4 wherein the solvent is selected from the group consisting of water and methanol.
 6. The method as set forth in claim 4 wherein the solvent is water.
 7. The method as set forth in claim 4 further comprising adding the wash liquor to the first liquid fraction to form a wash liquor/liquid fraction mixture and heating the wash liquor/liquid fraction mixture to a temperature of from about 50° C. to about 100° C. to evaporate solvent from the wash liquor/liquid fraction mixture.
 8. The method as set forth in claim 1 wherein the first etchant dissolves silicon particulate and metal particulate, the method further comprising: (c) separating the first solid/etchant mixture into a second liquid fraction and a second solids fraction, the second liquid fraction comprising the first etchant and dissolved silicon particulate and dissolved metal particulate, the second solids fraction comprising unspent abrasive grains and spent abrasive grains; (d) mixing the second solids fraction with water to form an aqueous suspension comprising unspent abrasive grains and spent abrasive grains; (e) separating the aqueous suspension into a recycle fraction and waste fraction, the recycle fraction comprising unspent abrasive grains and water, the waste fraction comprising spent abrasive grains and water; and (f) separating the recycle fraction into a liquid fraction comprising water and isolated unspent abrasive grains.
 9. The method as set forth in claim 8 wherein the aqueous suspension is separated into the recycle fraction and the waste fraction using a cyclone separator.
 10. The method as set forth in claim 8 further comprising drying the isolated unspent abrasive grains until the residual water content is less than about 1000 ppm.
 11. The method as set forth in claim 8 wherein the first etchant comprises a mixture of hydrofluoric acid and nitric acid.
 12. The method as set forth in claim 11 wherein the concentration of the hydrofluoric acid and the concentration of the nitric acid in the first etchant are both from about 10 to about 20 percent by weight.
 13. The method as set forth in claim 11 wherein the concentration of the hydrofluoric acid and the concentration of the nitric acid in the first etchant are both about 15 percent by weight.
 14. The method as set forth in claim 12 wherein at least about 6 liters of the first etchant is mixed per kilogram of the first solids fraction.
 15. The method as set forth in claim 1 wherein the first etchant dissolves silicon particulate or metal particulate in the first solid/etchant mixture, the method further comprising: (c) separating the first solid/etchant mixture into a second liquid fraction and a second solids fraction, the second liquid fraction comprising the first etchant and dissolved silicon particulate or dissolved metal particulate, the second solids fraction comprising unspent abrasive grains, spent abrasive grains and silicon particulate or metal particulate not dissolved in the first etchant; (d) mixing the second solids fraction with a second etchant to form a second solid/etchant mixture in which silicon particulate or metal particulate not dissolved in the first solid/etchant mixture is dissolved; and (e) separating the second solid/etchant mixture into a third liquid fraction and a third solids fraction, the third liquid fraction comprising the second etchant and dissolved silicon particulate or dissolved metal particulate, the third solids fraction comprising unspent abrasive grains and spent abrasive grains; (f) mixing the third solids fraction with water to form an aqueous suspension comprising unspent abrasive grains and spent abrasive grains; (g) separating the aqueous suspension into a recycle fraction and waste fraction, the recycle fraction comprising unspent abrasive grains and water, the waste fraction comprising spent abrasive grains and water; and (h) separating the recycle fraction into a suspending fraction comprising water and isolated unspent abrasive grains.
 16. The method as set forth in claim 15 wherein the aqueous suspension is separated into the recycle fraction and the waste fraction using a cyclone separator.
 17. The method as set forth in claim 15 further comprising drying the isolated unspent abrasive grains until the residual water content is less than about 1000 ppm.
 18. The method as set forth in claim 15 wherein the first etchant dissolves silicon particulate in the first solid/etchant mixture, the first etchant comprises water and a base selected from the group consisting of an alkali metal hydroxide and an alkaline earth metal hydroxide, and the second liquid fraction comprises the first etchant and dissolved silicon particulate.
 19. The method as set forth in claim 18 wherein the base is sodium hydroxide.
 20. The method as set forth in claim 19 wherein the concentration of sodium hydroxide in the first etchant is from about 6 to about 20 percent by weight.
 21. The method as set forth in claim 19 wherein the concentration of sodium hydroxide in the first etchant is from about 8 to about 15 percent by weight.
 22. The method as set forth in claim 20 wherein at least about 6 liters of the first etchant is mixed per kilogram of the first solids fraction.
 23. The process as set forth in claim 18 further comprising collecting hydrogen gas evolved during the dissolution of silicon particulate in the first solid/etchant mixture.
 24. The method as set forth in claim 15 wherein the second etchant dissolves metal particulate not dissolved in the first solid/etchant mixture, the second etchant comprises water and an acid selected from the group consisting of nitric acid and sulfuric acid, and the second liquid fraction comprises the first etchant and dissolved metal particulate.
 25. The method as set forth in claim 24 wherein the acid is sulfuric acid.
 26. The method as set forth in claim 25 wherein the concentration of sulfuric acid in the first etchant is from about 0.3 to about 1 percent by weight.
 27. The method as set forth in claim 25 wherein the concentration of sulfuric acid in the first etchant is from about 0.4 to about 0.8 percent by weight.
 28. A method for treating an exhausted slurry utilized in slicing silicon wafers from a silicon ingot, the exhausted slurry comprising a lubricating fluid, unspent abrasive grains, spent abrasive grains, silicon particulate and metal particulate, the method comprising: (a) separating the exhausted slurry into a first liquid fraction and a first solids fraction, the first liquid fraction comprising lubricating fluid suitable, the first solids fraction comprising unspent abrasive grains, spent abrasive grains, silicon particulate and metal particulate; (b) mixing the first solids fraction with water to form an aqueous suspension comprising unspent abrasive grains, spent abrasive grains, silicon particulate and metal particulate; (c) separating the aqueous suspension into a recycle fraction and waste fraction, the recycle fraction comprising unspent abrasive grains, silicon particulate, metal particulate, the waste fraction comprising spent abrasive grains, silicon particulate, metal particulate and water; (d) mixing the recycle fraction with a first etchant to form a first solid/etchant mixture in which the silicon particulate or the metal particulate is dissolved; (e) separating the first solid/etchant mixture into a second liquid fraction and a second solids fraction, the second liquid fraction comprising the first etchant and dissolved silicon particulate or dissolved metal particulate, the second solids fraction comprising unspent abrasive grains and silicon particulate or metal particulate not dissolved in the first etchant; (f) mixing the second solids fraction with a second etchant to form a second solid/etchant mixture in which the silicon particulate or the metal particulate not dissolved in the first solid/etchant mixture is dissolved; and (g) separating the second solid/etchant mixture into a third liquid fraction and a third solids fraction, the third liquid fraction comprising the second etchant and dissolved silicon particulate or dissolved metal particulate, the third solids fraction comprising unspent abrasive grains.
 29. A method for treating an exhausted slurry utilized in slicing silicon wafers from a silicon ingot, the exhausted slurry comprising a lubricating fluid, unspent abrasive grains, spent abrasive grains, silicon particulate and metal particulate, the method comprising: (a) separating the exhausted slurry into a first liquid fraction and a first solids fraction, the first liquid fraction comprising lubricating fluid, the first solids fraction comprising unspent abrasive grains, spent abrasive grains, silicon particulate, metal particulate and lubricating fluid; (b) washing the first solids fraction with a solvent to reduce the concentration of lubricating fluid in the first solids fraction and form a wash liquor comprising solvent and lubricating fluid; (c) adding the wash liquor to the first liquid fraction to form a wash liquor/liquid fraction mixture and heating the wash liquor/liquid fraction mixture to a temperature of from about 50° C. to about 100° C. to evaporate solvent from the wash liquor/liquid fraction mixture; (d) mixing the washed first solids fraction with an aqueous sodium hydroxide solution to form a first solid/etchant mixture in which silicon particulate is dissolved; (e) separating the first solid/etchant mixture into a second liquid fraction and a second solids fraction, the second liquid fraction comprising the aqueous sodium hydroxide solution and dissolved silicon particulate; the second solids fraction comprising unspent abrasive grains, spent abrasive grains and metal particulate; (f) mixing the second solids fraction with an aqueous sulfuric acid solution to form a second solid/etchant mixture in which the metal particulate is dissolved; (g) separating the second solid/etchant mixture into a third liquid fraction and a third solids fraction, the third liquid fraction comprising the aqueous sulfuric acid solution and dissolved metal particulate, the third solids fraction comprising unspent abrasive grains and spent abrasive grains; (h) rinsing the third solids fraction with water; and (i) drying the third solids fraction until the residual water content is less than about 1000 ppm.
 30. The method as set forth in claim 29 wherein the third solids fraction comprises less than about 7 percent by weight of spent abrasive grains.
 31. A method for treating an exhausted slurry utilized in slicing silicon wafers from a silicon ingot, the exhausted slurry comprising a lubricating fluid, unspent abrasive grains, spent abrasive grains, silicon particulate and metal particulate, the method comprising: (a) separating the exhausted slurry into a first liquid fraction and a first solids fraction, the first liquid fraction comprising lubricating fluid, the first solids fraction comprising unspent abrasive grains, spent abrasive grains, silicon particulate, metal particulate and lubricating fluid; (b) washing the first solids fraction with a solvent to reduce the concentration of lubricating fluid in the first solids fraction and form a wash liquor comprising solvent and lubricating fluid; (c) adding the wash liquor to the first liquid fraction to form a wash liquor/liquid fraction mixture and heating the wash liquor/liquid fraction mixture to a temperature of from about 50° C. to about 100° C. to evaporate solvent from the wash liquor/liquid fraction mixture; (d) mixing the washed first solids fraction with an aqueous sodium hydroxide solution to form a first solid/etchant mixture in which silicon particulate is dissolved; (e) separating the first solid/etchant mixture into a second liquid fraction and a second solids fraction, the second liquid fraction comprising the aqueous sodium hydroxide solution and dissolved silicon particulate; the second solids fraction comprising unspent abrasive grains, spent abrasive grains and metal particulate; (f) mixing the second solids fraction with an aqueous sulfuric acid solution to form a second solid/etchant mixture in which metal particulate is dissolved; (g) separating the second solid/etchant mixture into a third liquid fraction and a third solids fraction, the third liquid fraction comprising the aqueous sulfuric acid solution and dissolved metal particulate, the third solids fraction comprising unspent abrasive grains and spent abrasive grains; (h) mixing the third solids fraction with water to form an aqueous suspension comprising unspent abrasive grains and spent abrasive grains; (i) separating the aqueous suspension into a recycle fraction and waste fraction, the recycle fraction comprising unspent abrasive grains and water, the waste fraction comprising spent abrasive grains and water; and (j) separating the recycle fraction into a suspending liquid fraction comprising water and isolated unspent abrasive grains; and (k) drying the isolated unspent abrasive grains until the residual water content is less than about 1000 ppm. 